U.S. Pat. No. 3,959,531 to Schneble et al. indicates that iron in electroless copper depositions solutions should be below 25 mg/l, preferably below 10 mg/l in order to reduce extraneous copper deposits and avoid spontaneous decomposition of the plating solution. There is no teaching in U.S. Pat. No. 3,959,531 that trace quantities of codeposited iron affect the physical properties of the electrolessly deposited copper. U.S. Pat. No. 3,485,643 to Zeblisky et al. indicates that hexacyanoferrates added to electroless copper deposition solutions in concentrations up to 300 mg iron per liter will increase the plating rate and act as stabilizers and brighteners.
U.S. Pat. No. 4,650,691 to Kinoshita et al. also describes the use of hexacyanoferrates as stabilizers for electroless copper plating baths. Kinoshita et al. report that the hexacyanoferrates decompose in the plating bath and that the decomposition products inhibit further electroless copper plating and form precipitates in the bath. Kinoshita et al. indicate that the use of triethanolamine at one to three times the molar concentration of the hexacyanoferrate in the electroless bath will prevent destruction of the bath by inhibition of the plating reaction and the formation of precipitates.
U.S. Pat. Nos. 3,615,732; 3,615,722 and 3,615,735 describe the addition of Group VIII metal salts (Fe, Co, Ni, Pt and Os) to electroless copper plating solutions in combination with other additives to obtain improved ductility. U.S. Pat. No. 3,615,735 asserts copper alloys containing 0.05 to 2.5% of the Group VIII metals would provide superior ductility. The teaching of these three patents is that traces of Group VIII metals, including iron, in the copper deposit will improve ductility and the elongation under stress. The inclusion of iron to improve the ductility of copper deposits is in direct contradiction to, and teaches away from, the present invention described hereinbelow. Review of the more than 75 different examples in these three patents finds no ductile deposit containing iron. The only ductile copper deposits are in examples where the copper deposits contained traces of nickel and/or platinum as the Groups VIII metal.
U.S. Pat. No. 4,695,505 describes ductile, electrolessly deposited copper containing 74 to 280 ppm nickel. The elongation of 38 .mu.m (1.5 mil) thick copper was 11-15%. The copper plating baths contained 45-446 mg/l of nickel ions. The electroless plating bath cited had four stabilizers including potassium ferrocyanide. The potassium ferrocyanide concentrations were 60-1000 mg/l (17-293 mg iron/liter). The ductile copper deposits described in the example were obtained by discarding the plating solution every hour and replacing it with a brand new solution until the deposit thickness of 38 .mu.m was obtained. Discarding plating solutions on an hourly basis is not practical as a method of manufacturing copper deposits for printed wiring boards.
Standard Specification IPC-AM-372, ELECTROLESS COPPER FILM FOR ADDITIVE PRINTED BOARDS, The Institute for Interconnecting and Packaging Electronic Circuits, 7380 N. Lincoln Ave., Lincolnwood, Ill. 60646-1705 U.S.A. (1978) and Japanese Industrial Standard JIS H 8646 ELECTROLESS COPPER PLATINGS, Japanese Standards Association, 1-24, Akasaka 4, Minato-ku, Tokyo 107 Japan (1991) specify a copper purity of 99.2%. These specifications do not specify the trace metal concentrations in the copper deposit. They indicate only that the total trace metal (excluding silver) is below 0.8%.
There is a need in the printed wiring industry for high quality copper which will maintain reliable interconnections through the assembly process and the anticipated life of equipment containing the printed circuit. Printed wiring interconnections can fail by breaks in the copper conductive pattern caused by the stresses induced by the difference between the thermal expansion of the copper conductive pattern and the insulating substratum to which it is attached. The first set of thermal stresses occur during the assembly process when components are joined to the conductive pattern by soldering at temperatures from 200.degree. to 300.degree. C. In MIL 55110 D there is an attempt to address this problem by imposing a thermal stress test requirement on printed wiring boards. The thermal stress test requires the printed wiring boards to be conditioned by drying above 110.degree. C. for six hours to remove moisture. Then soldering flux is applied to the boards and they are thermally stressed by application of molten solder at 288.degree. C. for 10 seconds. After soldering, holes in the test board are microsectioned and examined with a low power microscope. Any evidence of a crack in the copper hole wall is considered a failure of the test. Since there may be more than one soldering cycle in the assembly of a printed circuit, some electronic equipment manufacturers have required suppliers of printed wiring boards to test the boards by repetitive thermal stress cycles at 260.degree. or 288.degree. C.
The second set of thermal stresses accumulate as thermal cycling occurs each time the equipment is turned on and off during use and the temperature rises from ambient temperature to the operating temperature of the equipment. Thermal cycling tests are used to simulate the cumulative effect of these stresses. A typical thermal cycling test is 30 minutes at -65.degree. C. followed by 30 minutes at 125.degree. C. A test board contains either 50 or 100 plated-through holes connected in series. More than 400 cycles with no open circuits and less than 10% increase in the resistance of the series of holes is considered good performance.
For subtractive printed wiring boards, the need for high quality copper has been addressed by standards for electroplated copper foils which specify the ductility and elongation for standard and high ductility copper foils, and by the development of baths and control methods for the baths for electroplating high quality copper for plated-through holes. For fully-additive and partly additive printed wiring boards, the need was not met. In spite of the need and a large number of groups working on this, no success was achieved.